1. Field of the Invention
The present invention generally relates to a method and apparatus of etch rate measurement. More specifically, the present invention relates to a method and apparatus of using optical temperature measurement as an in-situ monitor of etch rate during a plasma process.
2. Description of the Related Art
In order to obtain a reliable etching process, the etch rate (ER) of each plasma etcher during the manufacture of semiconductors must be closely monitored and controlled. Accordingly, it is becoming an important issue for the semiconductor industry to devise an efficient and accurate way of measuring the ER.
Conventional methods of ER measurement can be classified into two major categories. The first type involves etching directly on a test wafer; then the ER is obtained through comparing the differences in the thickness of the test wafer before and after etching. As a result, this procedure is both wafer- and time-consuming and thereby cannot be called in-situ. The accuracy of the ER measurement is also strictly dependent on the accuracy of the thickness measurement tool. On the other hand, a more advanced second type of ER measurement is typically known as in-situ depth/thickness monitoring. In this type of measuring tool, thickness change (i.e. ER) is monitored according to the principle of optical interferometry, which makes the test wafer unnecessary and thus saves time and wafer cost. Nevertheless, several factors are affecting the accuracy of the ER measurements that are carried out according to the conventional methods. In particular, the interference wave pattern depends critically on the refractive index of the film being measured. If there is any impurity or non-uniformity in the film, the wave pattern generated can be different from the normal one. Consequently, the measured ER might be different, too, and as a result it is possible that wrong decision might be made for the control of the etching process. Another issue presented by the conventional ER measuring method is that the light source used for monitoring a specific type of material usually has only a single wavelength. For monitoring a different type of material, another wavelength has to be chosen, which means that different etcher adjustments are needed.
Accordingly, it is an object of the present invention to provide a method and apparatus of in-situ ER measurement without the requirement of a light source.
Another object of the present invention is to provide a real-time ER measurement that will not be affected by the quality of the film being measured.
The present invention achieves the above-indicated objects by providing a method and apparatus of optical temperature measurement as an in-situ monitor of etch rate. First of all, according to the present invention, a plasma etching process is performed in a plasma etcher having a vacuum chamber. Then, an optical multi-channel analyzer (OMA) monitors a series of emission lines of a certain plasma species emitted from the vacuum chamber during the plasma etching process. Then, based on the intensity distribution of the emission lines detected, a computer computes and generates an optical temperature. Finally, the computer generates a relevant ER based on the optical temperature.
The emission lines are emitted due to the transitions between different energy states of a certain plasma species. These transitions may be between different electronic energy states, vibrational energy states, or rotational energy states, whereas the plasma species may be any one of the reactants in the plasma chamber such as CO, CO2, CF, CF2, SiF, C2, HF, etc.
The first major advantage of the present invention relates to the savings of time and money because the cost for test wafers and the time required for measuring the thickness of each test wafer can be excluded according to the method of the present invention.
The second major advantage of the present invention relates to the improved accuracy of the ER measurement that can be provided by the present invention. Since different plasma species emit different emission lines, the present invention can accurately obtain the optical temperature of the subject plasma species (i.e. the plasma in the excited state) which correlates to the real-time ER in the plasma chamber by selectively monitoring a plurality of emission lines that correlates to the subject plasma species.